In recent years, an airbag of which installation rate has been rapidly increased as one of automobile safety parts, prevents or protects the driver or fellow passenger in the following manner. Upon an automobile collision accident, a sensor senses impact, an inflator generates a gas at a high temperature and a high pressure, this gas rapidly develops an airbag, thereby, a body of the driver or fellow passenger, particularly a head, is prevented or protected from colliding against a handle, a front glass, a door glass, or the like.
Currently, not only a driver-side airbag or a passenger-side airbag against frontal collision of an automobile, a knee airbag for knee protection, a side airbag or a side curtain airbag against side collision, and an airbag against rear collision are being adopted. Further, in recent years, an airbag for protecting a pedestrian against collision has been known and the airbag installation portion continues to grow.
As the airbag installation rate is increasing, a lighter and more compact airbag module is a very important technique in order to secure fuel economy performance of and indoor space in an automobile. Among individual parts in such module, the airbag accounts for a significant ratio in terms of both volume and weight, so that a lighter and more compact airbag is always required.
To meet such requirement, in recent years, there has been an increasing demand for a lighter and more compact uncoated fabric to which a resin coating is not applied.
In order to make such uncoated fabric further lighter and more compact, the reduction in the amount of yarn used for a woven fabric has been studied by lowering the fineness of fibers that constitute the woven fabric, by reducing the number of stitches, or the like. This reduction, however, deteriorates the air permeability (increases the air permeability), so that the size of the inflator needs to be increased, resulting in increased volume and weight of the airbag module.
As a method for solving this problem, the following method has been known: The air permeability of the base fabric is lowered by setting the single yarn fineness that constitutes fibers smaller (Patent Document 1).
Other than the reduction of the single yarn fineness, there has been also known a method of forming the cross section of the single yarn into a flat shape to lower the air permeability (Patent Document 2). Patent Document 2 discloses that in the woven fabric using flat cross-section yarns, the longer diameters of the single yarn cross section are arranged in stack, so that the gap per unit cross section in the cross section of the woven fabric reduces, thereby achieving low air permeability.
Recently miniaturization of automobiles has been increased, and among development needs such as secured indoor space, visibility of various meters from the driver's seat, or reduction in weight of a vehicle body due to improvement of fuel efficiency during running of automobiles, in particular, a demand for a lighter and more compact airbag is growing rapidly.
An airbag used in an airbag system requires that a bag-like article that is obtained by cutting an airbag-use woven fabric into a specific shape and sewing the cut woven fabric, can be accommodated in a space as small as possible. In order to be accommodated in such a small space, a woven fabric which has low rigidity and is easily foldable is used.
As a general method for obtaining a woven fabric having low rigidity, a method of reducing the fineness of fibers that constitute a woven fabric, or a method of lowering the weave density of the fiber is known. These methods, however, have a difficulty in satisfying the performance necessary for airbags because the air permeability becomes higher even though the rigidity can be lowered. Therefore, as a method for lowering the air permeability of the woven fabric, a technique of using a fiber having a modified cross section has been proposed.
Patent Document 3 discloses a base fabric for an airbag which has a Y-shaped or a T-shaped cross section and is obtained by weaving fibers having a specific modification degree. It discloses herein that the base fabric is heated under a pressure and compressed to make the fabric texture packed, compressed, and compacted, and the single yarn fibers that constitute weaving yarns are deformed to make warps and wefts meshed with each other, so that low air permeability is achieved.
Patent Document 4 discloses the technique for achieving low air permeability, in which the present inventors have remarked that the squared-analogous cross-section yarns are difficult rolling and the woven fabric exhibit high shielding effect in the longitude and latitude directions, so that the airbag-use woven fabric is woven with fiber having substantially square-shaped cross section.